Effects of Cactoblastis cactorum on the survival and growth of North American Opuntia

Abstract

Cactoblastis cactorum is known for being both a biological control agent and an invasive pest of opuntioid cacti. The spread of C. cactorum in the southeastern United States may threaten the biological and physical integrity of desert, scrub, and coastal habitats. However, the effects of invasive species are known to vary spatially and temporally, and C. cactorum’s efficacy as a biological control agent varies considerably from region to region. Therefore, the long term effects of C. cactorum within its U.S. range are still uncertain. Marked Opuntia stricta (n = 253) and O. humifusa (n = 327) plants along the west coast of Florida were censused for 6 years to determine the effects of C. cactorum attack on survival and growth rate of plants, and to examine host species differences and the effects of plant size. 78.1 % of the Opuntia plants were attacked by C. cactorum during the 6 year study and the overall survival rate was 75.8 %. Plants attacked by C. cactorum were more likely to die than unattacked plants and a plant’s odds of surviving the 6 year period decreased as C. cactorum attack frequency increased. However, plants that survived the 6 year period showed, on average, positive growth and there was no significant difference in growth rates between surviving attacked and unattacked plants. O. stricta plants were more likely to be attacked at least once, were attacked more frequently, and were more likely to die after being attacked than were O. humifusa plants. Plant size did not predict plant survival, but larger surviving plants lost proportionally more pads over the 6 years than smaller surviving plants. Although C. cactorum should still be considered a threat, particularly for rare opuntioids, overall survival along the west central Florida coast is currently high and plants that are able to survive C. cactorum attack are not being reduced in size, possibly because they possess traits that render them more tolerant of C. cactorum damage. Our findings suggest that an assumption of severe negative effects of an invasive species, based on its effects in other regions or over short periods of time, may not always be justified.